Manufacture of dry contact rectifiers



Sept. 28, 1948.

B. B. GRACE ET AL MANUFACTURE OF DRY CONTACT RECTIFIERS 3 Sheets-Sheet 1 Filed Aug. 28, 1943 F/Gl. I I Z ni/111. 57!! Sept. 28, 1948. B. B. GRACE ETAL MANUFACTURE OF DRY CONTACT RECTIFIERS Filed Aug. 28, 1943 -3 Sheets-Sheet 2 r-ltlormiv Sqpt. 28, 1948. B. B. GRACE ETAL 5 MANUFACTURE OF DRY CONTACT BECTIFIERS Filed Aug. 28, 1943 Y s Sheets-Sheet 5 Patented Sept. 1 948 MANUFACTURE OF DRY CONTACT RECTIFIERS Bertram Barnett Grace, Leslie James Ellison, Hugh Spence Leman, Charles William Long, and Arthur Samuel Bridge, London, England, assignors to Standard Telephones and Cables Limited, London, England, a British company Application August 28, 1943,v Serial No. 500,438 In Great Britain February 15, 1943 8 Claims. (Cl..175-366) This invention relates to the manufacture of selenium rectifiers and more particularly to the electrical treatment thereof.

In the manufacture of selenium rectifiers it is well known that, after selenium has been applied 7 to a base plate and subjected to a suitable heat treatment, and a counter electrode has been applied, it is necessary to apply a voltage in the reverse direction of the rectifier for some time in order to build up a relatively high resistance in the reverse direction.

In order that this resistance in the reverse direction should be built up it has hitherto been necessary to increase the applied voltage in steps as the resistance increased, the fall of current being taken as an indication of the increase of the resistance.

By using rectified alternating current or pulsating direct current the voltage finally applied is about 20 volts.

In carrying this out on a commercial scale, a blast of air is blown past the. rectifiers to keep them cool. It has also been proposed to avoid undue heating and consequent damage to the rectifiers to apply a relatively high voltage to a rectifier for relatively short periods between which the rectifier is allowed to cool or positively cooled by an air blast. The relatively high voltage thus referred to was first 15 volts and then 20 volts.

Difllculties arise in applyin electrical treatment on a manufacturing scale in that although good results are sometimes obtained it frequently happens that the current remains at a given value for an inordinately long period or even refuses to fall at all. Moreover, even with air blast cooling some rectifiers are damaged by burning. Furthermore the necessity for constant attention to increase the voltage in steps is a disadvantage.

According to one'feature of the present invention the electrical treatment consists in applying a direct current source to a plurality of circuits in parallel, each circuit consisting of a. single rectifier and an individual resistance.

These resistances serve several functions which will be clear from the following,

A very important function of the resistance used in series with each rectifier is that it enables the final voltage to be impressed in the first place Without the necessity of raising it by steps. If E be the impressed voltage, 1' the reverse resistance of the rectifier, which varies and gradually rises,

R the resistance in series with the rectifier, VP the voltage across the rectifier at any instant, i the current through the rectifier at any instant, then As r increases during the treatment, n increases and increases and tends towards a final value of 1 when Vz =E.

Thus the presence of a resistance of appropriate value has the effect of automatically causing an increase of voltage across the rectifier as its resistance rises. In consequence the attention required to increase the applied voltage at intervals is dispensed with, thus rendering the process cheaper. It has been shown by experience that the total time required is not increased by the presence of these resistances whilst the total energy in watt-hours required is somewhat less.

Rectifiers as initially manufactured differ in their rectifyin properties and in treating a large number of rectifiers in parallel, initially poor rectifiers shunt the initially good rectifiers and so prevent the current necessary to build up a reverse resistance from passing through the latter.

Ii lombs per square centimetre. It follows that the time required for the electrical treatment can be reduce if the current density through each rectifier can be increased, the limiting factor being solely the rise in temperature of the rectlfiers and consequent burning out.

We have found, however, that the time of electrical treatment can be reduced to about one hour and a very high current density (of 110 ma. per square centimetre initially) be safely used if the rectifiers are efflciently cooled. In order to achieve this, according to a further feature of the present invention the rectiflers are supported during the treatment upon a fiat shelf through which cooling water is circulated, means for ensuring good contact with the fiat surface of the shelf being provided.

We have found that a selenium rectifier behaves as if it consisted of a rectifying resistance and an ohmic resistance in parallel, the value of the latter varying from 20,000 ohms per square cm., for a good rectifier to 3,000 ohms per square cm. for a poor rectifier. It is only the current through the rectifying resistance that is effective in building up the rectifying properties. As this reverse rectifying resistance is built up the portion of the current flowing therethrough decreases as the applied voltage increases and the portion of current flowing through the ohmic resistance increases. For low current densities therefore, the portion of current, flowing through the rectifying resistance may become too small to be effective and the phenomenon previously referred to may occur, viz: that the current through the rectifier disc may remain at a given value for an inordinately long time, or even refuse to fall at all. For high current densities, however, sumcient current passes through the rectifying resistance portion despite the shunting effect of the ohmic resistance to build up the resistance thereof during the whole time of electrical treatment. The resistance which, according to the invention, is connected in series with each individual rectifier, then serves to limit the current -through the shunt ohmic resistance, when the reverse resistance approaches its maximum, to control the heat losses in such oh'mic resistance. Thus the use of high current densities enables the time of treatment to be shortened in greater proportion than the increase in applied voltage, providedexcessive heat losses are avoided and efllcient cooling to prevent burning of the discs is used.

We have found that it is necessary to secure good heat conducting contact between the rectiflers and the surface of a flat metal channel through which cooling water is circulated. Circular rectifier discs are normally madev with a base plate which is slightly concave at the back and we have found it necessary in the case of discs of 84 and 112 mm. in diameter to use a pressure of between 20 and 40 pounds to ensure that the base plate is in contact with the cooling surface throughout the whole area of the former and thus to ensure adequate cooling of the rectifier. For smaller size discs a smaller pressure is sufficient. resistance of 170 ohms per square cm. area in series with each rectifier.

In order to keep the temperature drop in the cooling water for the rack small, the water is preferably circulated rapidly and a refrigerating plant used to cool the circulated water.

The best forms of electrical treatment and cooling apparatus known to us are described in the With this form of apparatus we use a following with reference to the accompanying drawings in which:

Fig. 1 is a perspective view of a portion of an apparatus for electrical treatment of circular selenium rectifiers of 84 mm. or 112 mm. diameter discs.

Fig. 2 is a front view, with a part in section of an apparatus for electrical treatment of lesser diameter selenium rectifiers.

Fig. 3 is a view partly in side elevation and partly in cross-section of another form of apparatus.

Fig. 4 is a view in cross section of still another form of apparatus.

Referring now to Fig. l, a fiat h'ollow shelf I0 is provided, through which cooling water is circulated from aninlet pipe II to an outlet pipe (not shown). Along the rear-of the upper surface of shelf i0 is a strip l2 having a curved front portion to allow rectifier discs I4 to be inserted partly thereunder, screws l3 protruding on the under side to locate the discs [4 in position. The hollow shelf I0 is supported from a framework (not shown) which supports a member ll! of channel section at a little distance above the shelf l0. Flexible metal bellows ll, one for each rectifier disc are supported on individual supporting and insulating plates l6 secured to the member II, the latter being out away'to allow of flexible tubing I8, connected to the bellows, passing through the member 15 tothe rear thereof. Each flexible tube I8 is connected in rear of the member I! to a pipe l9 connected to a suitable compressed air supply (not shown) a suitable cock or tap being provided to shut off the supply. The shelf l0 forms one common terminal of the electric supply and makes contact with the base plate of each rectifier Whilst an apron member 20 attached to each bellows makes contact with the counter electrode of its individual rectifier when compressed air is admitted to the bellows. Each apron member 20 is provided with a terminal tag 2| which is connected by a conductor 22 to one terminal of a switch 23 mounted in rear of the member l5. The operating member 30 of switch 23 is adapted when moved to one side to connect conductor 22 to a conductor 28 and when moved to the other side to connect conductor 22 to conductor 29, the middle portion being an offposition. Conductors 28, 29 are connected to clips 23 and 21 which are supported in an insulated manner on the framework and carry resistances 24 and 25 of different values, provided with a common terminal 3 I. The other side of the direct current supply used for the electrical treatment is connected to all the terminals 3i. The resistances 24 and 25 are of values adapted for rectifier discs of 84 and 112 mm. diameter and accordingly the front of member I 5 is suitably marked with 84 on one side of each switch member 30 and with 112 on the other side so that the operator can initially by manipulation of the switch members 30 connect in the correct value of resistance for each rectifier. The member I2 is mounted on side members 32, one only of which is shown, which members 32 are in turn held on shelf 10 by screws and butterfly nuts 33. Each screw passes through an elongated aperture 34 inmember 32 so that member I2 may be drawn forward such a distance that the screws l3 therein will properly locate the smaller of the two sizes of 'disc for which the apparatus is adapted.

flat shelf ill but help to cool the upper surfaces of the discs.

It is to be noted that the rectifier discs'to be electrically treated in this apparatus are constructed in accordance with U. S. Patent No. 2,314,104 granted March 16, 1943 to E. A. Richards and L. J. Ellison, entitled Metal rectifiers an area around the central aperture of each being covered with insulating varnish before the counter electrode is sprayed thereon. The pressure is exerted upon thispart at whicl. the counter electrode is not in contact with the selenium. These remarks also apply to the apparatus shown in Figs. 2 and 3. In all forms of the apparatus the resistances mounted in series with the rectifier discs are separated physically from these discs, so that heat developed in the resistances does not add to the diillculties of cooling.

Referring now to Figs. 2 and 3, which show apparatus used for the electrical treatment of rectifier discs of 67 mm. and 45 mm. in diameter, cooling water is circulated through a hollow flat shelf ill by means not shown. The shelf is provided on its upper surface with means for locating rectifier discs of either of two difi'erent sizes similar to the means shown in Fig. l.

A conical helical spring is provided for each rectifier, the base of each spring being anchored to a common frame member, 38 bolted to upright members '38 of the framework. The lower end of each spring is fixed to a rod 38 which carries a contact member 31. All the members .31 are held in the positions shown by a common member 40 which rests upon two cams 4i fixed on respective shafts 42 journalled in front and rear members of the framework. Fixed also to shafts 42 are cranks 43 finished with handles 44 by means of which shafts 42 may be rotated to raise and lower member 40. When the rectifier discs to be treated are in position upon plate 10, the member 40 is lowered and springs 35 thus allowed to press contact member 31 upon the centre portion of the rectifier discs. Each contact member 31 is connected by a strip45 to a conducting post 46 which in turn is connected by a flexible conductor to one terminal of a switch member (not shown). This switch member is similar to the member shown in Fig. 1 and is for the purpose of connecting a resistance of one or other value in serie with each rectifier.

Fig. 4 shows in cross-section an apparatus for electrical treatment of rectifier discs of 25 mm. or 18 mm. diameter. For discs of this size a lighter-pressure is sufficient and accordingly each disc 41 is pressed against the surface of the flat each end threeof. The other ends of leaf springs- 54 are secured to respective plates fixed to the framework. Extending forwardly of bar 53 at each end and secured to the bar is an arm '56 a depression of which is engaged by a lug 51 on a lever 58 pivoted at 59 to the framework. A rod 80 furnished with a handle Bi is attached to each lever 58. In the position shown in the drawing the fiat bar 53 is held down with each spring 48 passing on a rectifier disc 41 by the weight of levers 58 and rods 60. To release the rectifier discs so that they can be withdrawn. the levers 58 are pushed upwardly by means of rods 50 and handles 6| into the position of lever 58 shown in dashed lines until the lugs 51 are disengaged i'rom the depressions in levers 56 where- .upon these levers move under the influence of leaf springs 54 into the position shown dotted.

An insulating strip 62 is held on the upper surface of shelf l0 and formed with semi-circular portions to locate the rectifier discs in position. When levers 58 are pulled downwards to press springs 48 on to the rectifier discs, the springs 48 press downwith a rolling motion, which prevents a'ny scratching of the sprayed metal counter electrodes thereon.

The terminal 50 for each spring 48 is connected to the mid-point of two resistance spools 24 and 25. The other end of resistance 24 is connected to a bus bar 83 and the other end of resistance 25 to a bus bar 84. A common switch is provided for each shelf to connect the positive pole of the direct current supply either to bus-bar 83 or to bus bar 84 according to the size of the rectifier discs undergoing electrical treatment. The negative pole of the supply is connected to iile framework and thus to the fiat hollow plate What is claimed is:

1. Apparatus for the electrical treatment of selenium rectifiers comprising a channel for circulation of water, a flat surface for said channel on which the rectifiers are adapted to rest, means for circulating cooling water through said channel, means for holding said rectifiers in good heat conducting relation with the surface of said channel and contact means for connecting each of said rectifiers in series with an individual resistance in parallel to a common source of direct current.

2. Apparatus for electrical treatment of selenium rectifiers comprising a channel with means for circulating a cooling medium therethrough, a flat surface of said channel for supporting rectifiers, means for holding the rectifiers in good heat conducting relation with said surface, circuit means for connecting each of said rectifiers in series with an individual resistance in parallel to a common direct current source, said holding means comprising pressure applying means individual to each rectifier for pressing said rectifier against the surface of said channel and means for positioning the individual 'rectifiers beneath the pressure applying means.

3. Apparatus as claimed in claim 2 comprising means for positioning rectifiers of either of two different sizes beneath said pressure applying means, and switch means for connecting a resistance of either of two different values in series with the several rectifiers.

4. Apparatus as claimed in claim 2 in which said pressure applying means comprises a flexible metal bellows for each rectifier and means for admitting fluid under pressure to each bellows.

5. Apparatus as claimed in claim 2 in which said pressure applying means comprises a flexible metal bellows for each rectifier with means for admitting fluid under pressure to all said bellows simultaneously.

6. Apparatus as claimed in claim 2 in which said pressure applying means comprises a conical helical spring for each rectifier and means is provided for allowing all the springs to exert pressure on the respective rectifiers and means is 8 gagltgdferirgogggngrgggig rectiners simultane REFERENCES CITED 7, Apparatus as claimed in claim 2 i which The following references are of record in the said pressure applying means comprises for each file of this patent: rectifier a, spring metal strip bent into a closed 5 UNITED STATES PATENTS figure, all said strips being mounted upon a common bar, means being provided for locking said Number Name Dim! common bar in position to press each strip against 2,055,216 an Sept. 22, 1938 its corresponding rectifier. 1 9- PP et a1 Jan. 2 1941 8. Apparatus as claimed in claim 2 comprising 10 2,237,302 W ttke Apr. 8. 1941 a refrigeration plant located in the circulation 21261-725 PP 4. 1941 channel of said cooling medium. 3,379,187 Thompson Apr. 7, i942 BERTRAM BARNETT GRACE. LESLIE JAMES ELLISON. HUGH SPENCE' LEMAN. 1 CHARLES WILLIAM LENG.

ARTHUR SAMUEL BRIDGE. 

